The present invention relates to a laser resonator which is suitable for use in a laser regenerative amplifier, more specifically to preventing drops in efficiency of laser resonators due to thermal lensing of laser media.
FIG. 3 is a schematic view of a regenerative amplifier using a conventional laser resonator.
In FIG. 3, 103 denotes a planar reflective mirror, 104 denotes a laser medium, 105 denotes an excitation laser device, 106 denotes a laser, 108 denotes an excitation laser, 109 denotes a dichroic concave mirror, 110 denotes a concave mirror, and 111 denotes a laser input/output portion, the laser output portion 111 consisting of a polarizer 113 and a Pockels cell 114.
First, the operation of the regenerative amplifier shown in FIG. 3 shall be described.
The laser 106 is polarized in the direction of reflection from the polarizer 113 at the time of incidence. At the time of incidence of the laser beam, a voltage is applied to the Pockels cell 114, so as to rotate the polarization of the laser by 90° and take it into the laser resonator with the polarization transmitted by the polarizer 113.
In the laser resonator, a laser medium 104 is provided on the optical path of the laser between the planar reflective mirrors 103. The excitation laser device 105 shines an excitation laser 108 onto this laser medium 104. In order to allow the excitation laser 108 to provide an input, 109 is a dichroic mirror, selected such as to reflect the laser beam 106 which is to be amplified and transmit the excitation laser beam 108. The excitation laser device 105, mirrors 109 and 110 are positioned so that the areas in the laser medium 104 through which the laser beam 10 and the excitation laser beam 108 pass overlap, so as to allow efficient amplification of the laser beam 106.
The laser beam 106 which has been inputted to the resonator oscillates between the planar reflective mirrors 103 so as to pass through the laser medium 104 until reaching gain saturation. Then, voltage is applied to the Pockels cell 114 to rotate the polarization so as to be reflected by the polarizer 113, as a result of the which the laser beam 106 is reflected by the polarizer 113 to extract the amplified laser.
In this type of laser resonator, the thermal lensing effect often becomes a problem, particularly in solid laser media.
The thermal lensing effect as it pertains to laser media occurs when a portion of a laser beam passing through a laser medium is absorbed by the laser medium and consequently heats the laser medium. The resulting temperature gradient causes a corresponding refractive index gradient in the laser medium that gives rise to a lensing effect.
The thermal lensing effect normally forms the equivalent of a convex lens in the laser medium. Additionally, since this thermal lens changes the mode of the laser resonator, a deviation arises in the state of the laser spot diameter of the laser beam 106 and excitation laser 108 in the laser medium 104 of a regenerative amplifier as described above.
FIG. 4 shows a schematic representation of this, and is a schematic view for illustrating the state of laser passage through a laser medium in a conventional laser resonator. FIG. 4(a) shows the case where thermal lensing does not occur, and FIG. 4(b) shows the case where thermal lensing does occur.
In the state of FIG. 4(a) without thermal lensing, the laser beam 106 and excitation laser beam 108 roughly overlap inside the laser medium 104, and the excitation energy provided to the laser medium 104 by the excitation laser beam 108 can be efficiently used for amplification of the laser beam 106.
However, when thermal lensing occurs, there is a deviation between the states of the two laser beams, and when the state of overlap between the lasers in the laser medium 104 becomes poor as shown in FIG. 4(b), the amplification efficiency of the laser 106 is reduced. In the state shown in FIG. 4(b), the spot diameter of the laser 106 increases, as a result of which the laser 106 passes through much of the area in the laser medium 104 which has not been excited, thus reducing the efficiency. Additionally, only the area centered around the spot of the laser beam 106 is amplified, which results in reduced beam quality of the laser beam being used. Conversely to the case of FIG. 4(b), if the spot diameter of the laser 106 becomes smaller, then the laser beam 106 will pass through only a portion of the area of the laser medium amplified by the excitation laser beam 108. Therefore, only a portion of the energy which is invested by the excitation laser beam can be used, and this leads to reduced efficiency. Additionally, if the spot diameter of a laser is reduced in this way, the energy density increases and leads to damage in the laser medium.
While the effects of thermal tensing have been described here by taking as an example the spot diameter which is visually easy to grasp, the thermal tensing effect can also affect the modes of the laser beams inside the laser medium, thus also reducing the mode-matching between the laser beam and the excitation laser beam. This also leads to reduced efficiency and reduced beam quality.
This thermal lensing effect can be overcome by adequately cooling the laser medium, as long as the laser has a low output power. Additionally, in apparatus in which thermal tensing occurs, the state of the thermal lens remains mostly stable after reaching the steady state during operation, so that the shapes and positions of mirrors 109, 110 can be designed to optimally compensate for this state.
However, if the laser power is made higher, the thermal tensing effect in the laser medium will change during operation of the laser device or over repeated activity and suspension of the laser device, making it difficult to preemptively compensate for this effect in the design of the laser resonator. Specifically, the thermal tensing effect will vary with changes in the output power of the excitation laser beam, changes in the state of oscillation of the laser, and variations in the heat removing devices. If the state of the thermal lens in the laser medium changes considerably during the course of operation of the laser device, adjustments such as changes to the angles or movement of the positions of the mirrors 109, 110 must be made in order to compensate for this in conventional laser resonators. In order to do so, the operation of the laser device must be suspended, and time must be taken to perform work, thus making such compensation impractical.